I only had one problem with the BC hot lips (all plastic) mags...
I thought it was barrel droop, a tight bentz chamber with a possible burr, or just a shitty mag.
Looking at it closely, the bbl was not the problem at all, it was that the rounds sat a TINY bit too high in the mag. They would be at an OK level to feed in, but the round underneath (one still in mag) would be gouged by the bolt, causing it to jam on entry.
I pulled the bolt out and looked at the underside. There's 3 "guide" ridges along the bottom. My guess was that the bolt was being snagged by the middle ridge right at the edge of the bolt face. To be sure I didnt remove too much; I stuck an spent casing on the bolt. Then I took a dremel and slightly rounded off that pointy edge, giving it a taper. Very little material was removed.
Now the mags feed like a champ. To test it out, I shot 400 rounds as fast as I could pull the trigger and reload 2 mags. Had maybe a 1/2 dozen FTE's, but a VQ extractor should fix that problem. Other than that, they work awesome now. This was using cheap CCI-Blazer and mini-mags.
YMMV